Rotary furnaces



" Fem; 1955 JiBORGNON.

ROTARY FURNACES Filed Oct. 25, 1955 INVENTOR BY 0 AT TORN EY StatesROTARY FURNACES Jean Borgnon, Paris, France, 'assignor to DiffusionAlloys Limited, London, England, a British company Application October25, 1955, Serial No. 542,723

1 Claim. (Cl. 263-34) the finished treatment.

Furnaces of this kind generally comprise an externally heatedcylindrical retort rotated about its axis, which is horizontal orslightly inclined, at a speed so chosen that temperature regularity isobtained and the heated objects are deformed to the least possibleextent. Usually the retort is closed at one end by a fixed base and atthe other end by a movablecover, but is occasionally open at both endsand forms a kind of tunnel through which pass the objects being treated,the said objects flowing through the tunnel either through the agency ofa screw conveyor or merely due to the effect of gravity owing to theinclination of the retort.

When a closed retort is used, a furance of this kind should be adaptedto permit of all heat treatments being carried out in any kind ofatmosphere, but in most cases (white annealing, chromising, etc.) thedesired result is not obtained because the hot objects discharged fromthe retort come into contact with ambient air which, by oxidation,modifies the final conditions of the treatment. In addition, withfurnaces of this kind it is almost impossible to cool the treatedobjects at a controlled speed and in a controlled atmosphere unless thefurnace is stopped at each operation, with the result that considerableheat has to be expended to set the furnace going again.

This invention-has an object to develop the advantages of furnaces ofthis kind and to adapt the same to effect substantially continuouslycontrolled atmosphere treatments comprising cooling at a controlledspeed.

An improved rotary furnace according to the invention comprises a rotaryretort closed at one end, open at the other end and mounted on a fixedpivot and a removable retort which is a good heat conductor and theexternal dimensions of which are such as to permit the said removableretort to be engaged axially in the rotary retort, the said removableretort being closed at one end by a fixed base and at the other end by acover with the interposition of fluid-tight packing.

The removable retort forms a fluid-tight container rotating with therotary retort, and the temperature inside the said removable retort ismore evenly distributed than in the rotary retort because the walls ofthe removable retort are good heat conductors and because the rotaryretort acts as a distributing screen, being intermediate the flames orsimilar heating means and the removable retort.

Since the removable retort is fluid-tight, it'is possible to maintaintherein any desired atmosphere before, during and after treatment and,because of its mobility, to cool the removable retort and the contentsthereof acatent cording to any desired law of temperature decreasewithout the treated objects coming into contact with air. For thispurpose, the removable retort ismerely withdrawnfrom the furnace andplaced in a suitable enclosed space, while the furnace can immediatelyreceive a new charge of objects contained in another removable retort.This method makes it possible to obviate the considerable heat lossunavoidable heretofore when the furance had to be left to cool with theobjects therein.

In one advantageous embodiment of the invention, the removable retort isof a length sufficient to permit of the cover and fluid-tight packingbeing situated relatively far away from the very hot zone of the furnacein order to prevent any deformation likely to be detrimental tofluid-tightness, and the effective inner space can be limited to theheated zone in regular fashion by a false bottom on the cover.

Preferably the cover is provided with a gas outlet tube and with a cockwhich is kept open for the entire heating period to permit evacuation ofthe gas contained in the removable retort, which cock is closed as soonas it is required to carry out a treatment necessitating a flow of gasesinside the retort, the cover can also comprise an intake tube whichextends right down to the bottom of the retort so as to ensure that theentire inner space is scavenged by the flowing gases. A similar tube canbe provided if cooling is to be carried out in a neutral or reducingatmosphere, the last-mentioned tube then being connected to anappropriate external gas source. The fixedbase of the removable retortmay or may not comprise a closed inner tube forming a recess in whichengages a similar tube which is fast with the rotary retort and whichcan accommodate temperature controllingrneans.

When the. objects to be treated have been placed in a removable retortaccording to the invention, the same is introduced into the rotaryretort of the furnace which, if required, is previously heated. Uponterminationof the heat cycle to be effected, the removable retort iswithdrawn from the furnace either by tilting of the latter or by theremovable retort being drawn on a carriage or slide, the removableretort then being cooled outside the furnace under the conditionsdesired.

In all cases, and merely by operating one or two cocks, cooling of thetreated objects can be carried out under optimum surfacephysico-chemical conditions;

A removable retort according to the invention can be made of any desiredmaterial, but it should be noted that from the viewpoint of thermalefficiency the heat inertia of the material should be high. Thus, theretort accord ing to the invention is preferably made of metal with thinwalls which follow as intimately as possible the internal shape of therotary retort, but it is obvious that the invention is not limited tothis kind of wall and includes retorts of the kind hereinbeforedescribed and made of other materials such, for example, as refractorymaterials, more particularly silicon carbide which has a satisfactoryheat transfer coefficient at high temperatures.

The invention will now be further described by way of example withreference to the accompanying drawing, which is a view in axialdiagrammatic section of a furnace according to the invention.

Referring to the drawing, the furnace comprises a refractory brickchamber 1 closed at its forward end by an appropriate front 2 throughwhich extends the head of a rotary retort 3 which is of cylindrical formand which is adapted to rotate within the chamber 1 about asubstantially horizontal axis. For this purpose the retort 3 rests uponthree pairslof rollers 4 which are placed symmetrically in each pairwith respect to the axial vertical plane of the retort 3. The latterextends rearwardly by a shaft 5 extending with clearance through thatwall In of the furnace which is at the opposite end to the front 2.Keyed to the shaft 5 is a gearwheel 6 or any otherpequivalent meansadapted to permit the retort 3 to 'be rotatedby anysuitable drivingmeans.

retort 7 may be introduced into and removed from the retort 3by slidingaxially. The space between the walls of the two retorts should be smallto ensure efiicient heat transfer.

In the embodiment illustrated in the drawing, the shaft 5, which is fastwith the rotary retort 3, penetrates some distance into the'sarne and isclosed at its inner end. The shaft 5 therefore forms a housing orchamber 8 adapted to receive temperature-controlling means, theindications of which reveal, in conjunction with error tables, the exacttemperature inside the removable retort 7. That end thereof which isinside the furnace is closed by a base 9 having in its centre are-entrant part 9a which closely follows the shape of the end of theshaft 5. The opposite end of-the removable retort 7 has an outer radialflange 10. against which bears a closure cover 11 with the interpositionof suitable fluid-tight packing 12 such as an asbestos gasket which isclamped between the flange 10 and cover 11 through the agency of fixingbolts 13 on the latter.

It will. be seen in the drawings that the cover 11 is at some distancefrom the front 2 so as to be removed as -far as possible from the heatof the furnace and so as to obviate deformations of the said cover, suchdeformations being possible ways in which air may enter the retort 7. Inorder that that part of the retort projecting beyond the furnace, insidewhich part the temperature is less elevated, may not be used for thetreatments to be carried out, the cover 11 has in its interior a kind ofbox l l forming a false bottom, the length l of which is such that thewall 14a of the false bottom is located inside the chamber 1 andsubstantially at the same distance from the front 2 as is the base 9from the rearward wall 1a of the said chamber.

A tube 15 is welded to the centre of the false bottom 14a and extendsaxially in fluid-tight manner through the cover 11. The outer end of thetube 15 bears a .cock 16 adapted to allow the entry of ambient air intothe interior of the removable retort 7 or to isolate the same. nectedthrough the medium of rotary packing 17 of any known type. A tube 18 ofsmaller diameter than the internal diameter of the tube 15 is weldedaxially to the centre of the re-entrant part 9a on the base The tube 18is pierced with radial holes 19 in the vicinity of the part 9a, and isof such length that its free end penetrates a short distance inside thetube 15 when the cover 11 of the removable retort 7 is in position. Athird tube 20 is fixed axially to the centre of the cover 11 and is ofsuch diameter as to slide with very slight clearance in the tube 13, thefree end of the tube 20 being slightly set back from the radial orifices19 when the cover 11 is in position. The tube 29 is extended externallyup to a rotating packing (not shown) coaxial with the retort 7, and acock which can be the cock 16 if the same is of the multi-way type,permits the tube 20 to be connected to an appropriate gas source orenables such a connection to be broken.

4 As has already been stated, the retort 7 is made of a Preferably thetube 15 and cock 16 are intercon- I .4, wall of the retort 7 issubstantially in contact over its entire periphery with the inner wallof the rotary retort 3, so that heat transfer is readily efiected fromthe hot gases flowing in the chamber 1 to the objects to be reated whichare contained in the retort 7, the double screen which is formed by theconductive walls of the retorts 3 and 7 ensuring regular temperaturedistribution inside the retort 7. The rotation of the latter is effectedmerely by the two retorts rubbing together.

The furnace is operated as follows: The objects to be treated arecharged with or without treatment material or insulating material, intothe annular-space surrounding the tube 18 of the removable retort 7,charging being terminated when the level is reached corresponding to theposition of the false bottoml4a of the cover. The latter is placed inposition, the tube 20 first penetrating into the tube 18 which keeps apath free for the tube 20 through the centre of the objects to betreated, and the retort is hermetically sealed by tightening the nuts onthe bolts 13 after the asbestos gasket 12 has been positioned. The cock16 is opened so as to place the interior of the retort in communicationwith the ambient air, and the latter retort is so placed in the rotaryretort through the agency of a suitable means that the end of the shaft5 engages in the re-entrant part 9a of the bottom of the retort.

If the treatment to be carried out does not need a special internalatmosphere (ordinary annealing) or if this atmosphere is created in theheart of the mass being treated, the cock 16 is kept open until theretort reaches the treatment temperature and is then closed for theduration of the treatment to prevent any entry of air. If the treatmentneeds a special atmosphere, for example a reducing, halogenated or otheratmosphere, the internal tube 20 is connected to an appropriate gassource as soon as the retort 7 is positioned in the furnace and the cock16 is placed (or the special cock is opened it a special cock isprovided for gas inlet and outlet) in the position corresponding to theentry of a relatively rapid flow of gas which, upon issuing from thetube 20, passes through the orifices 19 in the base of the retort andexpels the air contained therein. The air issues from the retort throughthe annular gap between the tubes 15 and 18, while the tube 15, rotatingpacking 17 andcock 16 place the interior of the retort incommunicationeither with atmosphere or with an exhaust chimney or withan absorptive tank if the gas is a harmful one. Once scavenging has beeneflected, the speed of gas flow can be reduced to the value required forthe process being used.

When the treatment is terminated, the retort 7 is cut off from theatmosphere by suitable operation of the cock 16 or of the two cocks ifneed be, the retort 7 is withdrawn from the retort 3 and, according tothe cooling cycle to be carried out, is placed in a heated chamber, in amedium which is a poor heat conductor or just in the open air. Dependingupon requirements, the retort can be kept closed for the duration ofcooling or a flow of appropriate gas maintaining the desired atmospherein the heart of the retort can be caused to circulate therein.

It will be apparent from the foregoing that the improved rotary furnaceaccording to the invention readily enables all treatments to be carriedout on objects present in large quantities, for example, cementingfollowed by slow cooling permitting machining to be begun beforetempering, white annealing in a controlled atmosphere of metalssensitive to rapid cooling, the nitridation of steel objects, chromisingwith a return to ambient temperature in the complete absence of oxygen,etc.

It is obvious that if no gas flow is necessary for carrying the intendedtreatment into effect, the gas inlet tube 20 can be omitted, as can alsothe sheath 18 thereof fixed to the base 9a of the retort, the objects tobe treated then being positioned more readily.

By way of example of a treatment which can be carried out withoutdifficulty with the aid of the improved furnace according to theinvention, the results will be hereinafter given which were obtainedduring tests in a method of chromising steel objects.

Example The diffusion method used was that described in United Kingdomspecification No. 646,637, using chromous iodide formed in situ fromitselements charged in solid form. Since no gas was to flow in the retortduring this treatment, the retort did not comprise tubes 18 and 20 and asingle-way tap 16 was used. The retort was charged with steel articlessoft cemented to 0.12 to 0.13 mm. and with semi-hard steel articles with0.3 to 0.35% carbon. These articles were immersed in a chromisingmixture comprising 66 parts by weight of pulverised ferrochromium, 66parts by weight of chromium, 33 parts by weight of alumina and 0.2 partby weight of ammonium iodide. After the removable retort had beencharged and hermetically sealed through the agency of the cover 11 withthe false bottom 14, the removable retort was inserted in a conventionalrotary furnace previously heated to 900 C. in which the rotary retortrevolved at a rate of three revolutions per minute. The cock 16, whichwas open at charging, was closed as soon as the air contained in theretort had been partly expelled by expansion, and was kept closed forthe duration of heating and cooling. The furnace temperature wasmaintained at 900 C. for the whole operation, and after 4 hours heatingthe removable retort was withdrawn and cooled in the open air with thecock 16 closed, while another removable retort filled with articles tobe treated was placed in the furnace. After cooling, the cock 16 wasopened and the articles extracted from the retort had a silver-greyappearance but showed no trace of oxidation, thus demonstrating acomplete absence of air in the retort. The diffused layer had a uniformthickness of about 0.007 mm., the mean hardness of the layer being inthe region of 1600 Vickers. The uniform thickness of the difiused layerdemonstrates the uniformity of the internal atmosphere and the equaldistribution of temperature in the removable retort.

It is obvious that modifications can be made to the rotary furnacehereinbefore described, more particularly by the substitution ofequivalent technical means, without for that reason departing from thescope of this invention. More particularly, the retort 7 could bepositively driven by the retort 3, for example through the agency of theshaft 5. That end thereof located in the retort 3 could be of polygonalcross-section, in which case the re-entrant part 9a on the base of theretort 7 would be of matching cross-section.

1 claim:

A rotary furnace comprising a closed chamber, a first retort rotatablymounted therein and having an open end projecting beyond one end wallthereof, a second retort closely fitted in said first retort andinsertable and removable through the open end of the first retort, saidsecond retort having an open end projecting beyond the open end of thefirst retort, a cover closing the open end of the second retort, afluid-tight packing between said cover and said second retort, a tubepassing through said second retort, a cock in said tube for placing saidsecond retort in communication with external air and for cutting offsaid second retort therefrom, and a second tube having radial aperturesand extending through said second retort to a point close to the endthereof opposite said cover, a third tube in said second tube forconnecting said second tube to a source of gas, and a cock forcontrolling gas flow through said third tube.

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